The present invention relates to the field of electrical circuitry, and more particularly relates to layered circuit structures such as multilayer circuit boards, to components and methods utilized in fabrication of such structures and to methods of making the same.
Electrical components are commonly mounted on circuit panel structures such as printed circuit boards. Circuit panels ordinarily include a generally flat sheet of dielectric material with electrical conductors disposed on a major, flat surface of the sheet or on both major surfaces. The conductors are commonly formed from metallic materials such as copper and serve to interconnect the electrical components mounted to the board. Where the conductors are disposed on both major surfaces of the panel, the panel may have via conductors extending through the dielectric layer so as to interconnect the conductors on opposite surfaces. Multilayer circuit panel assemblies have been made heretofore which incorporate plural, stacked circuit panels with additional layers of dielectric materials separating the conductors on mutually facing surfaces of adjacent panels in the stack. These multilayer assemblies ordinarily incorporate interconnections extending between the conductors on the various circuit panels in the stack as necessary to provide the required electrical interconnections.
Electrical components which can be mounted to circuit panel structures include so-called xe2x80x9cdiscretexe2x80x9d components and integrated circuits which include numerous transistors on a single chip. Chips of this nature can be mounted to elements commonly referred to as xe2x80x9cchip carriersxe2x80x9d which are specialized circuit panel structures. A chip carrier may be incorporated in a package which is mounted to a larger circuit board and interconnected with the remaining elements of the circuit. Alternatively, the chip can be mounted directly to the same circuit panel which carries other components of the system. This arrangement is ordinarily referred to as a xe2x80x9chybrid circuitxe2x80x9d. Relatively large circuit panels are commonly made of polymeric materials, typically with reinforcement such as glass, whereas very small circuit panels such as those used as semiconductor chip carriers may be formed from ceramics, silicon or the like.
There have been increasing needs for circuit panel structures which provide high density, complex interconnections. These needs are addressed by multilayer circuit panel structures. The methods generally used to fabricate multilayer panel structures have certain serious drawbacks. Multilayer panels are commonly made by providing individual, dual sided circuit panels with appropriate conductors thereon. The panels are then laminated one atop the other with one or more layers of uncured or partially cured dielectric material, commonly referred to as xe2x80x9cprepregsxe2x80x9d disposed between each pair of adjacent panels. Such a stack ordinarily is cured under heat and pressure to form a unitary mass. After curing, holes are drilled through the stack at locations where electrical connections between different boards are desired. The resulting holes are then coated or filled with electrically conductive materials, typically by plating the interiors of the holes to form what is called a plated through hole.
It is difficult to drill holes with a high ratio of depth to diameter. Thus, the holes used in assemblies fabricated according to these prior methods must be relatively large and hence consume substantial amounts of space in the assembly. These holes ordinarily extend from the top or bottom of the stack. Even where interconnections are not required in the top or bottom layers, space must be provided for holes to pass through these layers so as to provide needed interconnections in the middle layers. Accordingly, substantial amounts of the available surface area on the panels must be allocated to the holes and to accommodate tolerance zones around the holes. Moreover, the electrical interconnections formed by depositing conductive materials in such drilled holes tend to be weak. The drilling method and the general nature of the laminates used therein is described, for example in Doherty, Jr., U.S. Pat. No. 3,793,469; and Guarracini, U.S. Pat. No. 3,316,618.
Various alternative approaches have been proposed. Parks, et al., U.S. Pat. No. 3,541,222; Crepeau, U.S. Pat. No. 4,249,032; Luttmer, U.S. Pat. No. 3,795,037; Davies, et al., U.S. Pat. No. 3,862,790, Fox, U.S. Pat. No. 4,954,878, and Zifcak, U.S. Pat. No. 4,793,814 all relate generally to structures which have metallic or other conductive elements arranged at relatively closely spaced locations on a dielectric sheet with the conductive elements protruding through the dielectric sheet in both directions. Such a sheet may be sandwiched between a pair of circuit boards and the circuit boards may be clamped or otherwise held together so as to provide mechanical engagement between conductive elements on the adjacent faces of the circuit boards and the conductive elements of the composite sheet. In each of these arrangements, the conductive elements, the composite sheet or both is resilient or malleable so as to provide for close engagement between the conductive elements of the composite sheet and the conductors on the circuit boards.
Beck, U.S. Pat. No. 3,616,532 and Dube, et al., U.S. Pat. No. 3,509,270 describe variants of this approach in which resilient elements are used with a fusible solder. These elements are mounted on insulating boards which are then stacked between printed circuit layers. The assembly is heated so as to melt the solder, thereby freeing the resilient elements so that the resilient elements and solder cooperatively form an interconnection between the adjacent circuit boards.
Evans, et al, U.S. Pat. No. 4,655,519 describes a connector with numerous strip-like contact springs disposed in holes in a flat dielectric body, together with other spring elements. The ends of the strips protrude from opposite surfaces of the body. These are adapted to compress when electronic elements are engaged with the body surfaces, so that the ends of the strips engage pads on the electronic elements. Walkup, U.S. Pat. No. 5,167,512 discloses a further system using springs disposed in holes in a dielectric body.
Grabbe, U.S. Pat. No. 5,228,861 describes a connector having a sheet-like dielectric body with numerous generally X-shaped spring contact elements, each having four arms, lying on a first side of the sheet. Two arms of each X-shaped element are bent inwardly toward the sheet, and extend through holes in the sheet so that the tips of these arms protrude above the second, opposite face of the sheet. The other two arms are bent away from the sheet, and hence protrude from the first surface. When the connector is placed between circuit panels, each X-shaped element is compressed between mating pads of the circuit panels, causing the bent arms to flatten and causing the tips of the arms to wipe the surfaces of the pads. After engagement, the contact is maintained by the resilience of the arms.
Bernarr, et al., U.S. Pat. No. 4,548,451 describes a connector or interposer having a sheet-like elastomeric body with crushable protrusions extending outwardly from oppositely-directed surfaces. Tabs formed from a metal-coated flexible film extend on both surfaces of the body, and overlie the protrusions. The tabs on opposite sides are connected to one another by vias. When the interposer is engaged between circuit panels, the tabs and posts are crushed between contact pads on opposing panels, and the tabs assertedly wipe the pads for more effective contact. The tabs are maintained in engagement with the pads by the resilience of the elastomeric sheet and the posts; there is no permanent bond formed.
Patraw, U.S. Pat. Nos. 4,716,049; 4,902,606; and 4,924,353 describe microelectronic connection schemes using deformable contacts protruding upwardly from a substrate. Each contact has a dome-like tip and a plurality of legs extending downwardly from the tip to the substrate. These contacts are formed by selective deposition of aluminum on pedestals of a fugitive material such as potassium chloride or a photoresist using a shaped mask. The pedestals are removed after deposition.
Dery, et al., U.S. Pat. No. 4,729,809 discloses the use of an anisotropically conductive adhesive material disposed between opposing sublaminates, the adhesive composition having sufficient conductivity across the relatively small spaces between conductors on adjacent layers to form an electrical interconnection therebetween, but having low conductivity across the relatively large spaces between adjacent conductors on the same surface so that it does not produce an unwanted lateral interconnection along one surface.
Berger, et al., U.S. Pat. No. 4,788,766 uses conductor bearing circuit lamina having hollow, eyelet-like via structures, each such via structure having a rim protruding vertically from the surrounding structure. Each such via structure is provided with a thin layer of a conductive bonding material. In making the multilayer structure, dielectric bonding films are interposed between the circuit bearing lamina. The dielectric films have apertures in locations corresponding to the locations of the eyelet structures, in the adjacent circuit bearing lamina. Thus, the upstanding rims of the eyelet structures can bear upon one another when the assembly is forced together under heat and pressure. The layers of conductive bonding material on the rims of the abutting eyelets are said to form bonds between the abutting eyelet structures.
Co-pending U.S. patent application Ser. No. 08/277,366 of Thomas H. DiStefano, et al., which is assigned to the same assignee as the present application, discloses an interposer having deformable contacts protruding upwardly from its surfaces. Each contact has a central axis normal to the surface and a peripheral portion adapted to expand radially outwardly from the central axis in response to a force applied by a pad on an engaged circuit panel. Thus, when the circuit panels are compressed with the interposers, the contacts expand radially and wipe across the pads. The wiping action facilitates bonding of the contacts to the pads, as by conductive bonding material carried on the contacts themselves.
Other structures for forming multilayer electronic assemblies are taught in Dux, et al., U.S. Pat. No. 5,224,265 and Ehrenberg, et al., U.S. Pat. No. 5,232,548, which use sublaminates made by depositing a dielectric material onto a perforated metal sheet, as by vapor-phase polymerization or electrophoretic bonding, to form a dielectric sheet with vias. The vias are filled with a flowable joining material such as a metal-loaded polymer. These structures are stacked and heated to join the vias into unitary vertical conductors.
Other multilayer assembly systems using flowable conductive materials to join structures in stacked elements are disclosed in Bindra, et al., U.S. Pat. No. 5,129,142. Still further improvements are disclosed in U.S. Pat. No. 5,282,312 of Thomas H. DiStefano, et al. The ""312 patent discloses as background certain lamination techniques or methods of making multilayer circuit assemblies using flowable conductive materials.
Despite these and other efforts in the art, there are needs for still further improvement.
The present invention addresses these needs.
One aspect of the present invention provides an interposer for making connections to electrical contact pads on a surface of a microelectronic element, such as a circuit panel, a semiconductor chip or other element having a contact bearing surface. The contacts define holes therein. The interposer includes a body having a first major surface, such that the body has horizontal directions parallel to the first major surface and vertical directions perpendicular to the first major surface. The interposer further has a plurality of conductors in the body, such as via conductors extending in or through the body. The interposer further has a plurality of contacts on or above the first major surface. Each of the contacts is permanently joined to one of the conductors, and extends radially outwardly from the conductor. Thus, each contact extends in a plurality of horizontal directions away from the conductor. Each contact has a periphery remote from the conductor and a central portion attached to the conductor. The contacts are adapted to deform so that the periphery of the contact will contract radially inwardly toward the central portion of the contact in response to urging the periphery of the contact against one of the contact pads on the surface of the microelectronic element, and inserting the central portion of the contact into the hole defined by the contact pad. As the microelectronic element is juxtaposed with the first surface of the interposer and forced toward the body, the contacts will wipe the contact pads of the microelectronic element. The wiping action removes oxides and other contaminants from the mating surfaces to provide an effective, low resistance electrical connection between the pads and the contacts and, in preferred embodiments, to facilitate bonding of the contacts and the pads.
The contacts are desirably arranged to deform so that the contact bends vertically downward, with the periphery of the contact moving toward the body, as well as contracting radially inwardly toward the central portion of the contact. In the initial, undeformed condition, the periphery of each contact may be spaced vertically above the body, with a gap between the periphery of the contact and the body. The body may be deformable at least near the periphery of each of the contacts, such that movement of the periphery downwardly causes the periphery to engage and deform the body. The body may have an adhesive layer covering the first major surface for adhering the interposer to the microelectronic element upon juxtaposition of the two components. The contacts may be disposed on the adhesive layer, and the adhesive layer may have a thickness beneath the contacts greater than the thickness of the adhesive layer between the contacts. Such a configuration supports the contacts during handling, while maintaining the contact in an extended position for assuring contact with contact pads on an adjacent microelectronic element.
The periphery of each of the contacts may be noncircular. For example, each of the contacts may include a plurality of tabs extending radially outwardly from the conductor, with each of the tabs having a tip remote from the conductor. The tabs of each contact may be disposed in a substantially symmetrical pattern about the juncture of the contact and the associated conductor. The contact may include four such tabs, in which case the substantially symmetrical pattern in a quatrefoil pattern. The contacts may be disposed in a substantially rectilinear grid having rows and columns, the individual tabs extending substantially diagonally with respect to the row and columns. The diagonal tabs permit a maximum contact density for a given tab size. The contacts may take other forms. For example, the tabs may be disposed in a star pattern with the tips of the tabs having a circumferential width less than a circumferential width of the tabs near the central portion. In this case, the tips may be pointed so as to enhance wiping between the contacts and the contact pads. Alternatively, the circumferential width of the tabs near the tips may be greater than the circumferential width near the central portion. Such a configuration promotes wiping along the edges of the contacts.
The contact may include a conductive bonding material adapted to facilitate electrical joining of the tabs to contact pads engaged with the tabs. The conductive bonding material may be selected from the group consisting of solders, brazing alloys, defusion bonding alloys and conductive materials incorporating a polymer.
The contact may be formed integrally with an associated conductor. The conductor may extend substantially perpendicularly to the first surface of the body. The conductors may be arranged in the body at a pitch of less than about 1 mm from center to center. The pitch may therefore be chosen to correspond to standard pitches used in present and future microelectronic element contact arrays.
The interposer may be two-sided. In that case, the body defines a second major surface facing in an opposite direction from the first major surface. At least some of the conductors are through conductors having first ends disposed adjacent to the first major surface and second ends disposed adjacent to the second major surface. At least some of the contacts are permanently joined to the first ends of the through conductors. Second end contacts are provided on or above the second major surface of the body, and are permanently joined to the second ends of the conductors. Each of the second end contacts extends radially outwardly from the associated conductor, and has a periphery remote from the conductor. Each of the second end contacts is adapted to deform so that the periphery of the contact will contract radially inwardly toward the central portion of the contact in response to engaging a second microelectronic element. The element is engaged by urging a periphery of the contact against a contact pad of the second microelectronic element and inserting the central portion of the contact into a hole defined by the contact pad. Upon engagement, the contacts wipe the contact pads of the second microelectronic element when the second microelectronic element is juxtaposed with the second surface and forced toward the body.
In another embodiment of the invention, an interposer is provided for making connections to a microelectronic element having pads defining holes therein. The interposer includes an interposer body having a first surface and a plurality of contacts on the body. Each of the contacts has a central portion extending through the first surface into the body, and a peripheral portion extending radially from the central portion on or above the first surface. The peripheral portion of each contact is adapted to bend downward toward the body. The central portion is adapted to remain substantially undeformed in response to a force on the peripheral portion directed downward toward the body.
The central portions of the contacts may be substantially cylindrical. The peripheral portions may include a plurality of tabs extending radially from the central portion. The interposer may further include a deformable layer between the first surface and the peripheral portions of the contacts to allow the peripheral portions to bend downward into the deformable layer upon engagement with the contact pads on the microelectronic element. The deformable layer supports the contacts during transport, while permitting them to deflect upon engagement. The deformable layer may be an adhesive layer. The peripheral portion of the contact may be spaced apart from the body, defining a gap therebetween.
In another embodiment of the invention, an interposer for making connections to electrical contact pads on a surface of a microelectronic element is provided. The interposer includes an interposer body have a first surface and a plurality of contacts on the body. Each of the contacts includes a central portion and a plurality of tabs extending radially outward from the central portion. Each of the tabs extends over the first surface, and is adapted to deform radially inward toward the central portion of the contact, in response to a force on the tab directed downward toward the body. The tabs may have asperities on top surfaces facing upward away from the body. The asperities engage and wipe a contact pad engaged with the contact, breaking through any oxidation that may be present on the contact pad in order to form a more reliable electrical contact.
In a method for making a multilayer circuit according to the present invention, a circuit panel and an interposer are stacked so that a first surface of the interposer confronts a surface of the panel. The interposer has a body defining the first surface, and conductors having ends adjacent or above the first surface. The interposer further has contacts on the conductor ends, including peripheral portions extending generally horizontally from the associated conductor ends. The peripheral portions of the contacts confront contact pads on the circuit panel. The contact pads on the circuit panel define holes facing the interposer.
The method further comprises the step of compressing the stacked panel and the interposer together. In this step, conductor ends enter the holes in the circuit panel, while the peripheral portions of the contacts are forcibly engaged with the contact pads. This causes the peripheral portion of each contact to be contracted radially inward toward the associated conductor end so that each peripheral portion moves horizontally with respect to the engaged contact pad, and wipes the contact pad.
The conductor ends in the interposer used in the method have a vertical position with respect to the first surface of the interposer. That vertical position remains substantially unchanged during the compressing step. Further, the compressing step may be performed so that the peripheral portion wipes the contact pad at an edge of the contact pad defining the hole. In that case, the peripheral portion may comprise a plurality of tabs extending radially outward from the conductor. The tabs have edges extending in a substantially radial direction, the edges of the tabs wiping the edges of the holes during engagement.
The holes in the contact pads used in the method may be circular or may alternatively be polygonal, in which case they are defined by a plurality of edges wherein adjacent ones of the edges form a plurality of vertices. Where the holes are polygonal, the peripheral portion of an adjoining contact may form a circular outer edge having a diameter larger than a minimum diameter of the polygonal hole. The circular outer edge wipes the edges defining the polygonal hole. The peripheral portion of the contact may alternatively have a plurality of tabs, each of the tabs corresponding to a single vertice of the polygonal hole. The tabs are aligned with the vertices so that during the compressing step, each tab wipes two adjacent edges of the hole.
The contact pad may be connected to a conductive via formed in the hole defined by the contact pad. During the compressing step, the peripheral portion of the contact may move vertically downward toward the body of the interposer.
The stacking step of the above-described method may further include the step of stacking a plurality of circuit panels and at least one interposer, in an interweaved, vertically alternating arrangement. In such an arrangement, an interposer is disposed between each pair of circuit panels, with oppositely-directed first and second horizontal surfaces of each of the interposers confronting surfaces of the panels. The conductors of each of the interposers include through conductors extending through the body of the interposer and having their ends and contacts at or above first and second surfaces of the interposer. The contact pads on the plurality of panels are interconnected with one another by the through conductors.
The method may also include the step of bonding the contacts to the contact pads. The bonding step may include momentarily heating the stacked panels and interposers to activate electrically conductive bonding material that interfaces between the contacts and the vias. The bonding material may be carried on the contact pads. The momentary heating step may cause a softening of the body of the interposer, thereby facilitating vertical movement of the peripheral portions of the contacts during the compressing step.
The method may include the step of causing an adhesive such as a dielectric bonding material to flow at interfaces between the interposers and the panels during the compressing step so as to fuse the interposer and the panel into a substantially unitary mass. The step of causing the dielectric bonding material to flow may include momentarily heating the stacked panels and interposers.
Each contact of the interposer used in the method may include a plurality of tabs extending generally radially outward in a substantially symmetrical pattern from the associated conductor end. During the compressing step, each tab is bent so that a tip of each tab remote from the conductor moves downwardly toward the interposer body and horizontally toward the conductor end. All the tabs in each of the contacts may be engaged with the same contact pad during the stacking step.
In the method of the invention, the first surface of the interposer may be coated with an adhesive such as a pressure-sensitive adhesive. The adhesive is activated during the compressing step to bond the first surface of the interposer to the surface of the panel. Further, wiping of the peripheral portions of the contacts on the contact pads may cause at least some of the peripheral portions to be friction welded to corresponding ones of the contact pads.
In a method of making a circuit assembly according to another aspect of the invention, a circuit panel and an interposer are stacked in a vertically superposed arrangement so that a first horizontally extensive surface of the interposer confronts a first horizontally extensive surface of the circuit panel. A plurality of electrically conductive contacts on the first surface of the interposer confronts contact pads on the circuit panel. The contact pads face the interposer and define holes in the contact pads. The stacked panel and interposer are then pressed vertically so as to forcibly engage the contacts with the contact pads. The contacts are caused to deform so that at least a portion of each contact enters a hole defined by one of the contact pads and at least a portion of each contact engages the contact pad and moves horizontally inward and vertically downward with respect to the first surface of the interposer, thereby wiping the contact pad. The contacts may wipe the contact pads at edges defining the holes.
This process of the invention may further include the step of bonding each contact to the engaged contact pad. The bonding step may include the step of activating bonding material at interfaces between the contacts and the vias by momentarily heating the stacked interposer and circuit panel. The bonding material may be present on the contacts, the contact pads or both prior to the stacking step.
The interposer in this method may include a dielectric layer at the horizontally-extensive surface. The contact in that case includes a portion in contact with the dielectric layer and bearing on the engaged contact pad during the compressing step. The momentary heating softens the dielectric layer to facilitate vertical downward deformation of the contacts. The bonding may alternatively or additionally include compressing the stacked panel and the interposer so that portions of the contacts wipe the contact pads with sufficient force to friction weld the contacts to the engaged contact pads.
In a method of making a microelectronic interposer according to the invention, a body is provided having a first surface. A temporary layer is provided over the first surface of the body. Apertures passing through the body and the temporary layer are formed, and a layer of an electrically conductive structural material is deposited in each of the apertures and over the temporary layer proximate the aperture. The conductive structural material forms contacts on the temporary layer that extend into the apertures. The temporary layer is then removed, leaving the contacts with outwardly flaring peripheral portions that are spaced vertically above the first surface of the body.
An adhesive may be deposited on the first surface of the body before providing the first temporary layer. The temporary layer is then provided over the adhesive layer, and the adhesive layer is left exposed after the temporary layer is removed. The adhesive layer may be a thermoplastic or b-staged adhesive. In a preferred embodiment, the adhesive may be a material selected from the group of polyimide and polyetherimide (PEI). The temporary layer may be formed from a metal selected from a group consisting of aluminum, tin and nickel.
The body may also define a second surface opposite the first surface. In that case, the method further comprises providing a second temporary layer over the second surface, with the apertures passing through the second temporary layer, as well as the first temporary layer and the body. The electrically conductive structural material is deposited so as to extend over the second temporary layer proximate the apertures to thereby form second contacts.
In another method of making a microelectronic interposer according to the invention, a body defining the first surface is provided, and a first compliant layer is formed over the first surface. Apertures are then formed passing through the body and the compliant layer. A layer of an electrically conductive structural material is then deposited in each of the apertures and over the compliant layer proximate the aperture to form contacts with outwardly flaring peripheral portions on the compliant layer. The compliant layer may be an adhesive. The compliant layer may be partially etched so that the compliant layer has a thickness under the contacts greater than a thickness between the contacts.